Designing for Stormwater Management with Permeable Interlocking Concrete Pavements (PICP)
Permeable interlocking concrete pavement (PICP) systems are engineered to manage stormwater runoff at the source, reducing the burden on municipal infrastructure and improving water quality. Unlike traditional dense-graded bases that rely on compaction for water shedding, permeable systems utilize open-graded aggregates to facilitate immediate vertical infiltration. For architects and engineers in California and Arizona, specifying these systems requires a precise understanding of the relationship between the paver units, the jointing material, and the multi-layered open-graded base. Acker-Stone offers several solutions for these applications, including the Aqua-Via I, Aqua-Via II, and the 4 x 12 AquaLiña, which are specifically designed to meet environmental consciousness and best management practices for runoff control.
The Anatomy of an Open-Graded Base
The performance of a permeable system is dependent on the void space within the aggregate layers. An open-graded base consists of crushed stone with no "fines" (small particles or dust), allowing water to flow freely through the gaps between the stones. This structure serves two purposes: providing structural support for vehicular or pedestrian loads and acting as a reservoir for temporary water storage.
Sub-Base and Base Layers
The foundation of the system typically begins with a sub-base of larger crushed stone, such as No. 2 or No. 57 stone. This layer provides the primary reservoir capacity. Above this, a base layer of smaller aggregate, often No. 57 stone, is used to bridge the gap between the coarse sub-base and the finer setting bed. These layers must be mechanically stabilized through compaction to ensure structural integrity without clogging the void spaces that allow for infiltration.
The Setting Bed
The bedding layer is the final leveling course upon which the pavers are placed. For permeable applications, this is typically a 1.5 to 2-inch layer of No. 8 crushed stone. This size is small enough to provide a smooth laying surface for units like the 4 x 12 AquaLiña or Aqua-Via Trio, yet large enough to prevent the material from migrating down into the larger voids of the base stone.
Joint Aggregate and Infiltration Rates
The joints of a permeable paver system are the intake valves for the entire assembly. Unlike standard pavers that use polymeric sand or fine masonry sand, permeable systems require open-graded jointing stone, typically No. 8, No. 89, or No. 9 stone.
Products like Aqua-Via I and Aqua-Via II are engineered with specific spacer bars that create consistent joint widths. These joints, when filled with the correct aggregate, allow water to pass through the surface at rates significantly higher than most natural rainfall events. It is critical to specify clean, washed stone for the joints to prevent the introduction of fines that could lead to premature clogging.
Structural Design for Pedestrian and Vehicular Loads
While the primary goal is infiltration, the system must also support the intended traffic. The 4 x 12 AquaLiña, for example, is developed specifically for pedestrian and light vehicular applications. When designing for these loads, the thickness of the open-graded base must be calculated based on both the structural requirements of the expected traffic and the hydrological requirements of the local design storm.
Common Permeable Paver Patterns
For units like the 4 x 12, popular installation patterns include:
- Herringbone: Provides the highest level of interlock, ideal for vehicular areas.
- Running Bond: A classic linear aesthetic often used for walkways.
- Stacked Bond: A modern, clean-lined approach for pedestrian plazas.
Technical Specifications for Permeable Systems
| Component | Material Specification | Typical Depth |
|---|---|---|
| Surface Unit | Aqua-Via I, Aqua-Via II, 4 x 12 AquaLiña | 60mm - 80mm |
| Joint Filler | No. 8, 89, or 9 Crushed Stone | Full depth of paver |
| Bedding Course | No. 8 Crushed Stone | 1.5" - 2" |
| Base Course | No. 57 Crushed Stone | 4" - 6" |
| Sub-Base | No. 2 or No. 57 Crushed Stone | Varies by design |
Infiltration and Subgrade Considerations
The final stage of the infiltration design is the interaction with the native subgrade. In well-draining soils (Hydrologic Soil Groups A and B), the system may be designed for full infiltration. In poorly draining clay soils (Groups C and D), the system may require an underdrain to slowly release filtered water into the storm sewer system after the peak of the storm has passed. Geotextiles may be specified to separate the open-graded base from the native soil, provided they are selected for high permittivity to ensure water flow is not restricted.
Specify It
To ensure your project meets local stormwater requirements while maintaining structural durability, utilize the AckerStudio AI to generate custom patterns and technical drawings for your permeable specifications. We recommend requesting physical samples of Aqua-Via I or 4 x 12 AquaLiña to verify joint widths and aggregate compatibility for your specific site conditions.